Add STATIC_ASSERT_INCOMPLETE_CLASS to enforce include minimality.

Add enforcements against `Dictionary` for `ustring.h` and two for `Dictionary` and `String` from `array.h`.

Author: Ivorforce

SConstruct

@@ -632,6 +632,7 @@ if env["strict_checks"]:
 
 # Run SCU file generation script if in a SCU build.
 if env["scu_build"]:
+    env.Append(CPPDEFINES=["SCU_BUILD_ENABLED"])
     max_includes_per_scu = 8
     if env.dev_build:
         max_includes_per_scu = 1024

core/string/ustring.cpp

@@ -30,6 +30,8 @@
 
 #include "ustring.h"
 
+STATIC_ASSERT_INCOMPLETE_CLASS(Dictionary);
+
 #include "core/crypto/crypto_core.h"
 #include "core/math/color.h"
 #include "core/math/math_funcs.h"

core/typedefs.h

@@ -440,3 +440,23 @@ inline constexpr bool is_zero_constructible_v = is_zero_constructible<T>::value;
 #define GODOT_MSVC_WARNING_POP
 #define GODOT_MSVC_WARNING_PUSH_AND_IGNORE(m_warning)
 #endif
+
+template <typename T, typename = void>
+struct is_fully_defined : std::false_type {};
+
+template <typename T>
+struct is_fully_defined<T, std::void_t<decltype(sizeof(T))>> : std::true_type {};
+
+template <typename T>
+constexpr bool is_fully_defined_v = is_fully_defined<T>::value;
+
+#if !defined(SCU_BUILD_ENABLED) && !defined(DEV_ENABLED) && !defined(DEBUG_ENABLED)
+/// Enforces the requirement that a class is not fully defined.
+/// This can be used to reduce include coupling and keep compile times low.
+/// The check must be made at the top of the corresponding .cpp file of a header.
+#define STATIC_ASSERT_INCOMPLETE_CLASS(m_type) \
+	class m_type;                              \
+	static_assert(!is_fully_defined_v<m_type>, #m_type " was unexpectedly fully defined. Please check the include hierarchy of '" __FILE__ "' and remove includes that resolve the class.");
+#else
+#define STATIC_ASSERT_INCOMPLETE_CLASS(m_type)
+#endif

core/variant/array.cpp

@@ -30,6 +30,9 @@
 
 #include "array.h"
 
+STATIC_ASSERT_INCOMPLETE_CLASS(Dictionary);
+STATIC_ASSERT_INCOMPLETE_CLASS(String);
+
 #include "container_type_validate.h"
 #include "core/math/math_funcs.h"
 #include "core/object/script_language.h"